1. Field of the Invention
The present invention relates to a frequency-variable band-pass filter and a method for making a photonic crystal used in the band-pass filter.
2. Description of the Related Art
A band-pass filter incorporating a one-dimensional magnetic photonic crystal as a magnetooptic member has been available in the related art. For example, Japanese Unexamined Patent Application Publication No. 2002-90525 (“'525 document” hereinafter) describes a one-dimensional magnetic photonic that includes two dielectric multilayer films in which the dielectric materials are alternately laminated with each thickness thereof being regular and an irregular layer (defective layer) composed of a magnetic material.
The structure of the one-dimensional magnetic photonic crystal described in '525 document is shown in FIG. 1.
As shown in FIG. 1, a magneto-optic member 10 is constituted by the one-dimensional magnetic photonic crystal and includes two periodic dielectric multilayer films 13 and 14 in each of which two kinds of dielectric substances (dielectric thin films) 11 and 12 of uniform thickness are alternately laminated and a magneto-optical thin film (magnetic substance) 15 which is provided between the two 1
The periodic dielectric multilayer films 13 and 14 serve as reflecting mirrors of a Fabry-Perot resonator, and the thickness of each of the dielectric substances (dielectric thin films) 11 and 12 is designed so that the optical length (product of actual film thickness and refractive index) is equal to λ/4 (where λ is the wavelength of light). The optical length of the irregular layer (defective layer) constituted by the magnetic substance (magneto-optic thin film 15) in which localization of light occurs is set equal to mλ/2 (m is a positive integer).
However, an optical isolator including the magneto-optic member described in '525 document has a frequency characteristic determined by the thicknesses of the two dielectric substances. Accordingly, the pass band will be fixed if the optical isolator is configured to serve as a band-pass filter. Thus, this technology has not been applied to band-pass filters of communication devices such as multiband transmitters and the like.